With the recent advancement in information technology, electronic devices are becoming smaller sized, lighter and more portable. As a result, demand on batteries with higher energy density as power source of these electronic devices is also increasing. The lithium secondary battery is a battery capable of satisfying this requirement and studies are being actively carried out thereabout. The lithium secondary battery comprises a cathode, an anode, an electrolyte providing a path for lithium ions between the cathode and the anode and a separator. Electric energy is produced through oxidation-reduction reactions as the lithium ions are intercalated into and deintercalated from the cathode and the anode.
A non-aqueous electrolyte used in a lithium secondary battery generally includes an electrolyte solvent and an electrolyte salt. However, the electrolyte solvent is decomposed on the electrode surface or co-intercalated between the carbonaceous anode layers during charging and discharging of the battery, thereby collapsing the anode structure and damaging stability of the battery.
It is known that such problems can be solved by a solid electrolyte interface (SEI) film formed on the anode surface owing to reduction of the electrolyte solvent during initial charging of the battery. However, the SEI film is generally insufficient to serve as a film for continuously protecting the anode and its life and performance are deteriorated after repeated charging and discharging of the battery. In particular, the SEI film of the existing lithium secondary battery is thermally unstable. Thus, if the battery is operated or kept at high temperature, the SEI film may collapse easily with time due to electrochemical energy and thermal energy. As a result, the battery performance is deteriorated at high temperature. Particularly, gas such as CO2 is continuously generated due to collapse of the SEI film, decomposition of the electrolyte, or the like. Consequently, the internal pressure and thickness of the battery are increased.
In order to solve these problems, a method of using, for example, vinylene carbonate as an electrolyte additive for forming a film on the anode surface was proposed. Although vinylene carbonate exhibits superior storage performance at high temperature and superior cycle performance, output performance is not at low temperature.
Recently, the lithium secondary battery is used for various purposes, from general electronic devices to various applications including electric vehicles. In this regard, demand on a high-output battery is also increasing. Accordingly, a battery capable of providing high output not only at high temperature but also at low temperature is necessary.